The use of conventional switch mode power supplies, or converter circuits, for charging capacitive loads, such as a camera photoflash capacitor, is known. U.S. Pat. No. 6,518,733 to Schenkel et al., entitled “Circuits and Techniques for Capacitor Charging Circuits,” describes various features of such a circuit. For example, a switch control scheme is described in the '733 patent with which the main converter switch is controlled based on the transformer primary winding current and the secondary winding current. Specifically, the switch on time is controlled in response to the sensed primary winding current and the switch off time is controlled in response to the sensed secondary winding current. The above-referenced patent also describes sensing the converter output voltage from the primary side of the transformer with a comparator in order to determine when the capacitor is charged to the desired level. One-shot circuitry is used to disable the output of the comparator for an interval at the beginning of each off time of the switch since the voltage spikes occurring when the switch shuts off are not indicative of the actual output voltage. Also described is control circuitry to conserve power by ceasing the delivery of power to the capacitor load once the desired output voltage is reached. An interrogation timer activates the power delivery circuitry at a predetermined time after it has been deactivated in order to maintain the capacitor output load in a constant state of readiness. In some embodiments, the control circuitry disables both the power delivery circuitry and the measuring circuitry once the capacitor voltage is at the desired level in order to further reduce power dissipation.
In a related U.S. Patent Publication No. 2004/0130299, also entitled “Circuits and Techniques for Capacitor Charging Circuits,” an alternative switch control scheme in described with which the main converter switch is controlled based on the transformer primary winding current and the primary winding voltage. Specifically, the switch on time is controlled in response to the sensed primary winding current and the switch off time is controlled in response to the sensed primary winding voltage.